TITLE: GCN CIRCULAR NUMBER: 41782 SUBJECT: GRB 250911A: COLIBRÍ optical counterpart candidate DATE: 25/09/11 03:50:26 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx> Camila Angulo (UNAM), Sarah Antier (OCA), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM): We imaged the field of the Swift GRB 250911A (Palmer et al., GCN Circ. 41781) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed at 2025-09-11 02:41 UTC (1.2 hours after the trigger) and obtained 60 second exposures in the r and z filters. The data were analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detect an uncatalogued source consistent with the BAT 3.0 arcmin error circle (Palmer et al., GCN Circ. 41781) at: RA(J2000) = 17:35:00.05 = 263.75021 degrees Dec(J2000) = -16:51:14.40 = -16.8540 degrees with an uncertainty of 0.5 arcsec. The preliminary magnitudes derived for that source are: r = 19.19 +/- 0.08 z = 18.46 +/- 0.10 We suggest that this source is the optical counterpart of the GRB. Further observations are ongoing. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/41782. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...